Fix #392, 'setMenu of null' with multiple instances

[keforbes]

This is more of a workaround than a true fix but it works fine. Basically, we're setting the rebuild-menu event listener on ipcMain but there is only a single instance of ipcMain across all Oni windows. When one of the instances receives a rebuild-menu event from one of the other instances it would try calling mainWindow.setMenu() and get this error.

My fix is to just ignore the event if mainWindow is null. So it avoids the error but I feel like in the future we should find a way to isolate the Oni instances a little better.

[bryphe]

My fix is to just ignore the event if mainWindow is null. So it avoids the error but I feel like in the future we should find a way to isolate the Oni instances a little better.

Sounds good for now. At least this will suppress the error. I think that the event object returned from ipcMain has a WebContents instance through the sender event property of the object it returns. It looks like we need the BrowserWindow instance though - but maybe we can derive that from the WebContents. That way, we could pass in the BrowserWindow that is relevant for the event to rebuildMenu.

I'm also wondering if later there is a way to handle more of the menu logic in browser/src instead of building it from main.js - I think we might be able to call those methods directly from the browser process without needing to ipc out, and that might help us out here.

In the meantime this will definitely help alleviate that pain point - thanks for the fix, @keforbes !